Perhaps not exactly what you are looking for, but there is a bundle (by Hilton Lipschitz) for specifying themes in plain text files as comma separated values (CSV): <https://github.com/hiltmon/textmate-theme-csv> 

Additional blog writeup here: <http://hiltmon.com/blog/2013/02/22/multiple-themes-in-textmate-2/>

I have a fork of the code with some additional features over at <https://github.com/persquare/textmate-theme-csv>

In particular, you can set the gutter color(s), and it can do rudimentary highlighting of color codes by assigning them a scope with the color code appended at the end. In practice, it is only useful if you, like me, like to work with a limited set of colors since they need to be added to the theme used when editing themes... (makes your head spin, I know ;) Attached screenshot should clarify what it looks like.

While not as easy to use as a GUI, plain text files allows you to easily track (and backtrack) changes to your theme(s).

/Per